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#1 |
Registered User
Join Date: Apr 2015
Location: Tennessee
Posts: 17
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Truck has started running rich & terrible MPG
Had my '79 sidestep for just over a year now. Drove it about 3 hours home when I bought it and it did great. No gas mileage issues at all.
Here's my symptoms:
I did not grow up working on cars. I actually bought this truck to begin learning how to fix them. The engine is the original 350. The carb is a Rochester Quadjet. I bought a vacuum gauge today, but haven't hooked it up yet. Plan to do that tomorrow. From what I've read, it appears the carb could need tuning, the ignition timing could be off, or maybe a vacuum leak? Based on the symptoms I'm experiencing, what would you recommend I check first? The symptoms popping up so quickly after running great for so long is what I'm struggling with right now. Appreciate your help!
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1979 C10 - Silverado with sidestep |
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#2 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: Truck has started running rich & terrible MPG
At 65, 5000 rpms? Sounds like you were in second gear.
Which would be the case if the vacuum line from the back of the carb down to the transmission broke. The tranny thinks you are at WOT. And the carb is struggling to add enough gas to keep up with the vacuum leak. I'd start with the vacuum line to the tranny. Inspect it where it connects to the vacuum modulator on the transmission, and all the way back. Replace the rubber portion on general principles.
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Rich Weyand 1978 K10 RCSB DD. |
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#3 |
Registered User
Join Date: Dec 2014
Location: Northern California
Posts: 422
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Re: Truck has started running rich & terrible MPG
Is the choke opening up? Agreed with above, but also sounds like it may be stuck on fast idle.
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#4 |
Registered User
Join Date: Apr 2015
Location: Tennessee
Posts: 17
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Re: Truck has started running rich & terrible MPG
Hey guys, thanks for the comments. Here's a few more details I was able to piece together today. One thing that jumped out to me, I'll put at the very bottom of this post. Might be nothing, not sure.
Here's a couple additional pics of the carb: In the 1st photo, I hooked the vac gauge to the blue inlet on the left (video above). I tried hooking it up to the blue inlet on the front, but nothing happened. https://drive.google.com/open?id=0B0...GZSZnpkZVdybms Passenger Side: https://drive.google.com/open?id=0B0...1QzM1loQmpVYmc Front view. The blue cap is the inlet that I hooked the vac gauge to and got no reading. It appears that are 2 tuning screws at the bottom of the carb. The one on the right is much further out that the one on the left. Could this be my problem? https://drive.google.com/open?id=0B0...WFlSUxZRWNVNnM Dumb question, but nonstop asked if my choke is opening up. How do I check this? Thanks guys!
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1979 C10 - Silverado with sidestep |
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#5 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: Truck has started running rich & terrible MPG
The choke plate is the metal plate standing on its edge in the front barrels. It is shown as open in these pics, so it is opening up. Whether it is closing when cold is another matter.
Your vacuum advance is connected to the ported vacuum on the front upper left (looking back) of the carb. Better is to hook it to manifold vacuum on the passenger side, the one sticking out the side with the blue cap on it. Move the cap to the one in the front to close it off. The two screws in front are the idle mixture screws. Before you mess with them, screw them each all the way in (not tight!) while counting turns. This will let you knew where you are so you can get back to where you are now. Then back them back out. They probably should be out about two turns. You can start from two turns and then try this. Let the car get to operating temp. Back the screws out until you get the highest vacuum reading. Back each screw the same amount, do 1/2 turn at a time and let the engine adjust. When you no longer see the vacuum or RPM increase, then turn each screw in 1/4 turn. Adjust your curb idle speed back to the spec with the idle adjustment screw on the throttle linkage.
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Rich Weyand 1978 K10 RCSB DD. |
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#6 |
Registered User
Join Date: Apr 2015
Location: Tennessee
Posts: 17
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Re: Truck has started running rich & terrible MPG
Rich, I really appreciate your help!
Went out and followed your directions. Turns out the mixture screws were set at about the same that I put them at. Then I adjusted the curb idle. I backed it out 3/4 of a turn. Here's what happened to my RPM's In Park - went from 1700 to 1300 In drive - went from 1300 to 1000 At 60mph - 5000+ due to the road I went out on, I was only able to get up to 50mph tonight, but RPM's topped out around 3600. It seems to shift gears and drive much smoother now. Think I might need to adjust the timing a little bit, but I'm really happy with how it's running right now. Hopefully MPG gets better. Again, REALLY appreciate the help. This is probably the "biggest" work I've done on the motor of a car. Learned a ton - which is why I wanted an old truck.
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1979 C10 - Silverado with sidestep |
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#7 |
Registered User
Join Date: May 2010
Location: Crossville, TN
Posts: 142
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Re: Truck has started running rich & terrible MPG
I only mention this because it happened to me once.
On the back of that cheap tach you have, should be some sort of switch. This selects 4, 6 and 8 cylinder engines. You may have to Google the brand to find out more info, but I had one set for a 6 cylinder by mistake, and got crazy high readings like you. I might be all wet, but it's worth a look... Jerry |
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#8 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: Truck has started running rich & terrible MPG
Yup. Aftermarket tach should have 4. 6. 8 selector. If that's wrong....
As for timing, I assume you have a timing light. A cheap one will work. Like $20-30 at any auto parts store. Disconnect the vacuum to the distributor. You can plug the hole, but you don't need to just to set the timing. Measure your timing at idle. You are probably around 8* BTDC. You should be able to go up to 12-16* BTDC. More is better as long as it doesn't knock when you get on it at low speed going uphill. Make sure you clamp the distributor back down tight. I assume in recommending this that you moved the vacuum advance line to the side port on the Qjet.
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Rich Weyand 1978 K10 RCSB DD. |
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#9 |
Registered User
Join Date: Jan 2013
Location: Toronto, ON
Posts: 458
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Re: Truck has started running rich & terrible MPG
Look to see that the little tang is dropping.
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Found my dad's old truck http://67-72chevytrucks.com/vboard/s...d.php?t=563321 Newspaper Article Publishing the quest find my dad's old truck http://www.theglobeandmail.com/globe...ticle12494281/ |
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